1. Field of the Invention
This invention relates to lubricating oil compositions, particularly to lubricating oil compositions useful as functional fluids in systems requiring fluid coupling, hydraulic fluid, and/or lubrication of relatively moving parts. In a preferred embodiment, this invention relates to a lubricating oil composition useful as the functional fluid in automatic transmissions, particularly automatic transmissions used in passenger automobiles.
The trend today is towards longer and longer periods of time between servicing of the modern passenger automobile. This trend includes servicing of the automatic transmission. Automobile manufacturers, for the convenience of their customers, are seeking to extend the time between fluid changes in the automatic transmission to greater and greater mileages.
Automatic transmission fluids are required to have a variety of desirable characteristics besides acting as a satisfactory fluid coupling or torque converter. Among these are allowing the transmission to shift smoothly, allowing the transmission to lock up during a shift from one speed to another within a certain specified period of time, and lubricating relatively moving parts such as bearing surfaces and clutch plates.
An automatic transmission is a complicated piece of machinery. It includes a turbine drive unit with a torque converter and one or more clutches which are engaged and disengaged automatically by an intricate hydraulic control unit. In a typical automatic transmission, the clutches are made up of alternating steel plates and steel plates faced on both sides with a friction material such as compressed paper.
To achieve a smooth shift, the clutch plates are not abruptly engaged, but are compressed together at a controlled rate, with pressure varying with speed and torque. Therefore, for a finite measurable period of time, the friction facings and steel surfaces are in relative motion until complete engagement occurs. The time lapse between when shifting begins and relative motion between the plates ceases is called the time to lock up.
This time to lock up is an important specification to be met in qualifying an automatic transmission fluid for use in the transmission of an automobile manufacturer. In order to not cause a great strain on the drive train and obtain a smooth shift, maximum and minimum times to lock up are specified.
An automatic transmission should not emit noises when it shifts. This problem is most noticeable in certain transmissions; especially when they are used with high output engines (e.g., 400 cubic inches displacement and larger). It occurs during manual shifting (e.g., Park to Drive, Park to Reverse, Drive to Reverse, etc.) of the transmission. The noise emitted is generally described as a "clunk."
The functional fluid used in automatic transmissions is subjected to very severe conditions of use. The temperature of the automatic transmission fluid under normal operating conditions will reach 275.degree.F. Under more severe conditions, such as during climbing hills, trailer towing, stop-and-go traffic in the metropolitan areas, etc., the fluid temperature can increase significantly above this, up to, for example, 325.degree.F and higher. In addition, the fluid is constantly being pumped and agitated, thereby being brought into intimate contact with the atmosphere within the automatic transmission. Fresh air and atmospheric moisture are constantly introduced through the transmission housing breather tube. Under these conditions of high temperature and thorough mixing, the fluid tends to be oxidized, forming undesirable contaminants in the fluid, and modifying or impairing the desirable characteristics of the fluid.
The degradation products which are produced during use cause the characteristics of the functional fluid to change. The smoothness of the shift can be lost and the time it takes the transmission to lock up during a shift from one speed to another increases. As will be appreciated, when the lock-up time increases, the clutch facings are in relative motion to each other for a greater period of time, thereby allowing for the possibility of greater wear, higher clutch facing temperatures, and greater heat input to the fluid.
Eventually, the time to lock up will increase until it becomes too long to be acceptable. Prior to this point, the automatic transmission fluid must be changed to avoid permanent damage to clutches and/or bands.
In addition to the possibility of permanent damage to friction members, the degraded oil contains various contaminants which can either coagulate and settle out or plate out as a film throughout the transmission. This is particularly detrimental in the small passages and close-fitting spool valves of the hydraulic control unit where a small amount of deposit can significantly change the size of the openings, cause sluggish valve movement thereby changing flow rates and pressures which, in turn, can markedly affect the performance of the entire transmission. In addition, if particles or lumps form in the fluid, they could completely block small openings such as in screens or filters and totally inpair the function of the transmission.
What is needed for the modern day automatic transmission is a fluid which is stable over an extended use interval, retains its shifting characteristics over this interval and reduces or eliminates noise generation and emission in the transmission.